1. © Crown copyright Met Office Extended-range forecasts for onset of the African rainy seasons examples and ideas for future work Michael Vellinga, Richard Graham and Alberto Arribas, Met Office

2. © Crown copyright Met Office Outline 1.Motivation 2.Onset forecasts Criteria and definitions Longrange forecast skill: level and sources Example forecasts 3.What next: thoughts and discussion

3. © Crown copyright Met Office Motivation

4. © Crown copyright Met Office Sahelian rainfall

5. © Crown copyright Met Office Motivation Rain-fed agriculture in Africa vulnerable to fluctuations in timing of arrival time of rains, as well as rainfall amounts: a)Long-range decision window: choice for slow or fast cropping varieties/crops agricultural logistics (field preparation, mobilisation of work force,..) b)Medium-range to-monthly range decision window: optimal sowing time A strong information need among regional stake- holders, not well addressed at present (also non- agricultural applications)

6. © Crown copyright Met Office Observed onset W Africa (JAS) GPCP mean 1996-2009 Calendar date (month/day)

7. © Crown copyright Met Office Observed onset W Africa (JAS) GPCP standard deviation St Dev (days)

8. © Crown copyright Met Office Observed onset (SOND) GPCP standard deviation – Substantial! St Dev (days)

9. © Crown copyright Met Office Onset forecasts Definitions, skill and examples

10. © Crown copyright Met Office Onset definitions: two classes 1.Local criteria: time of local rainfall threshold exceedance (e.g. Sivakumar 1988, AGRHYMET) 2.Regional criteria: Large-scale key changes to regional meteorology (wind field, convection, OLR, MSE, etc) We have developed trial longrange forecasts using such definitions, presented at African RCOFs in 2011/12 Formulated as tercile probabilities (‘earlier than average’, ‘average’ or ‘later than average’)

11. © Crown copyright Met Office Four indicators to define monsoon onset in W Africa 1.Northward shift of rainfall maxima from Gulf of Guinea coast to Sahel 2.Northward shift of OLR minima 3.Establishment of monsoon cell circulation and BL MSE gradient 4.Local rainfall threshold exceedance These four indicators together inform our onset forecast Intended users are NMHCs and regional Centres/RCOFs, not individula farmers

12. © Crown copyright Met Office Local indicator used in our trial forecasts : Describe temporal evolution with rainfall accumulations between 21 May-27 October Scale by long-term average season total accumulation. Example: 1 member Time Percentage of season total rainfall onset= 20% Average time of onset Heavy line: accumulated precip. from climatology Thin line: accumulated precip. for individual year example: early onset in individual year

13. © Crown copyright Met Office Forecast skill for onset ROC score for earlier-than-average onset 25 April hindcasts, i.e. ~2 months’ lead indicator 1)

14. © Crown copyright Met Office Skill scores for onset over W Africa in ENSEMBLES Using standard definition for ‘regional’ onset (Fontaine Louvet 2006) ENSEMBLES hindcasts (1979-2005) in red circle From Vellinga et al 2012 Clim Dyn

15. © Crown copyright Met Office HadiSST and GPCP-derived onset Observed teleconnection Source of predictability: June SST GloSea4 teleconnection Forecast skill (anom. corr.) For more details see: Vellinga et al 2012 Clim Dyn

16. © Crown copyright Met Office Other regions… GHA (SON) SE Asia (MJJ) S Africa (OND)

17. © Crown copyright Met Office Seasonal forecasts for onset Examples from 2011/12

18. © Crown copyright Met Office 2012 Onset W Africa (JAS) Glosea4 6 May forecast: probability for early (lower tercile) onset Observed onset anomaly 2012 (CPC/FEWS)

19. © Crown copyright Met Office 2011 Short Rains onset GHA (SON) Observed (CPC/FEWS) anomaly Glosea4 18 July forecast: probability for early (lower tercile) onset

20. © Crown copyright Met Office 2012 Short Rains onset GHA (SON) Observed (CPC/FEWS) onset anomaly (days) Glosea4 22 July forecast: probability for early onset

21. © Crown copyright Met Office Discussion points: Possible interaction with S2S Strong demand for this type of onset forecast information Not only at seasonal lead times, but also monthly Could set up multi-model prediction page for onset diagnostics: East/West/southern Africa What detail is possible at monthly lead time (range of dates)? What is the role of MJO/other tropical systems in delaying/advancing onset? How predictable are these? Is there multi-model agreement about years with high vs. low predictability for onset etc? Other aspects of intraseasonal variability - how accurately can a dry spell or wet spell be predicted in week 3?